from my experience with nano-/pico-tank fish species
1) all my tanks are under 6 gal. most are 1 gal or less. my general rules of thumb, I guess, could be said to be:
1a) species or "community"? 1st decide based upon size & personality/temperament of fish
1b) maintainable water quality/water conditions/tank set up/plant mix (at approx. 50/50) trump size/number formulae.
1c) for "community" tanks... no more than 2-4 types of "big enough to see" living creatures (fish, snails, shrimp) in tanks 2.5 gal or less. no more than 5-7 in 5+ gal tanks.
2) concur with recommendation on mosquito rasbora. generally many of the "nano" (VERY small/1" or less) rasbora seem to do well so long as you tend the water & pick the right tankmates. two other families that I've found ideal as the show fish: anabantoids (bettas, smaller gourami, badis/dario, day's paradise fish), and killies (Nothobranchius, Aphyosemion & soon Rivulus).
3) by size/termperament:
3a) small size fish:
- most VERY small rasobora (mosquito, exclamation point, neons, dwarf/pygmy, etc.). NOT cross-band/emeralds though. they really like to suddenly dart long distances and can't in small tanks. keep in groups of 7, 12, or more. be careful since most of these fish require gentile tankmakes. small gourami (choco's, pygmy and honey/C chuna) seem to me to be too agressive for all of them when the gourami are breeding. I am looking to set up a licorice gourami nano rasbora tank. I'll let ya know how that one goes.
- any dwarf corys (hasborus, hastus, pygmeaus, etc.). keep in groups of 3, 5, 7, or 10.
- scarlet badis (aka dario dario ...). 1 M:2-3 F. or 3-5 M:0 F. never keep only 2 M. tank will need lots of landmarks, clear demarcations (like "hedge" of tall plants) & "caves" / hiding places (e.g., under anubias nana leafs) for the M.
- bumblebee goby. 1 per 5 gal (or less). never keep only 2 M.
3b) medium size fish:
- espi harlequin rasbora. I have a LFS that I visit often that has had them in a 3 gal tank for months. that tank was my inspiration for trying mosquito rasbora.
- white clouds. I've raise them from fry in a 1 gal (based on what I'd read up to then), but I don't think I'd put them in anything less than a 5 gal anymore based on my experience. I heard that they are/were originally/can be mountain stream fish so think mountain stream loaches and think of how little fun they may have in a small tank.
- tetra. neon (I have diamond heads), golden phantom, belgian flag all seem to do well in 2.5-5 gal. some tetra species seem to want to swim about; others seem to swtich between dart & hang about. keep in groups of 3, 5, 7, or 10.
- blue-eye rainbow. furcata, gertrudes, & signifers in 5+ gal tanks. keep in groups of 3 (1 M:2 F). water MUST be very high in KH. dissolved baking soda at water changes seems to work well . mine seem to want to breed all the time. now if I can only get the eggs to hatch.
- smaller gourami. choc's, pygmy, honey (the natural/original variety is remarkably beutiful in a tank they like). choc's seem to be A-O-K as a trio (1 M; 2 F). pygmys are NOT; esp'ly once they start to pair up for breeding. for my honey/C chuna, I have just a pair always & I've had the F kill the M and vice verse while getting to know each other before breeding.
- otocinclus. they may be a bit like white clouds. solitary is ok, but don't forget that they like groups. NO "monkey fish" loaches. 2-3 Kuhli Loaches seem to work A-O-K for 2.5-5 gal.
3c) large size fish:
- Killies. I've had really good expriences with N. eggersi, A. australae (Cape Lopez killie), rocket killie, and A. gardneri nigerianum. each time they have made their 1-2.5 gal tank into breeding tanks. My rivulus tank is next.
- Betta splendids, other betta. ...including Day's Paradise Fish.
Both bettas and killies seem like to me to be VERY inquisitive as well as shy or even agressive at times. Sometimes they hide. Sometimes they are more like cats. do not put tail nippers with either of them. Day's Paradise Fish seem, so far, to me to be never aggressive with tankmates, but otherwise much like my killie and betta.
hope that helps.
finally, Ray, I am curiuos how your shell dwellers pico-tank turns out. Please lemme know how it goes.
I've thought of a Lamp. ocellattus or L. signatus species (or maybe with Tanganika killie) tank at the 1.2-3 gal size. I understand both M & F like to "own" more than 1 shell each. I've also heard they like their tanks to be taller than wide. I was thinking of a tall large vase/glass jar approach. I've not figured out how to build the rock facia yet for the back either. I've also seen L. ocellatus enjoying themselves in swimming upstream against a small power head down by the substrate in a larger tank. not sure how to replicate that in small tank yet.